Generally, digital conference arrangements sum information samples representing, for example, the level of speech from each conferee and then distribute the resulting sample to each conference port. In a teleconference, the communication between conferees should sound like a typical two-party connection; however, this is usually not the case when the conferees communicate with each other over transmission facilities having different amounts of loss which introduce differences in speech levels heard by the conferees. These differences between speakers, referred to as contrast, can be quite annoying to the conferees.
One of the more common ways to minimize this contrast is to utilize a gain control or level adjustment circuit to compensate for transmission loss. One prior art arrangement involves one or more conferees transmitting a test signal to the conference circuit and then adjusting the gain of an amplifier connected to the conferee's port to compensate for that leg's transmission loss. Another prior art arrangement utilizes subscriber station transitions (on/off-hook) to adjust the signal level of conferee input samples. Both arrangements have disadvantages in that they only adjust the signal level of samples for variations in transmission facility losses. They do not dynamically adjust the signal level for speaker variations such as, for example, voice strength, distance from a microphone, etc. In addition, when speech level is used to establish priorities for the selection of simultaneous speakers, these speaker variations put a speaker with a soft voice or low volume at a disadvantage in the selection process.
Another problem with the prior art is a noticeable change in signal level which may be accompanied by an annoying "click" when a conference port is abruptly enabled and disabled from the conference arrangement.